Conventionally, a small two-stroke engine has been well known, which includes a cylinder with an exhaust port and a scavenging port that allows scavenging air (fresh charge) containing at least fuel and air to be supplied to the inner side surface of the cylinder opposite to the exhaust port (hereinafter referred to as “schnurle-type two-stroke engine”).
Generally, such a schnurle-type two-stroke engine is configured to open and close the discharge port and the scavenging port by the reciprocating motion of a piston to allow the scavenging air to flow into the cylinder and to allow the exhaust gas to be discharged from the cylinder.
This schnurle-type two-stroke engine has a simple structure, and therefore part of the scavenging air having flowed into the cylinder via the exhaust port is often discharged from the exhaust port without being combusted by a spark plug, which is called “blow-by phenomenon”. In this case, deleterious components contained in the exhaust gas discharged from the exhaust port increase, and then are discharged from the exhaust port. This causes a problem that the charging efficiency deteriorates and the engine output is reduced.
To address the problem, for example, a schnurle-type two-stroke engine including a piston top surface on which a groove having an approximately arc cross section is formed, has been proposed (see Patent Literature 1).
With this schnurle-type two-stroke engine disclosed in Patent Literature 1, the groove formed on the piston top surface can allow the scavenging air (containing residual gas) having flowed from the exhaust port to successfully tumble. As a result, the scavenging air exhibits swirl motion in the cylinder, and therefore it is possible to prevent the above-described blow-by phenomenon from occurring.